Connected: An Internet Encyclopedia
RFC 1583 - 13.6. Retransmitting link state advertisements

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### 13.6. Retransmitting link state advertisements

Advertisements flooded out an adjacency are placed on the adjacency's Link state retransmission list. In order to ensure that flooding is reliable, these advertisements are retransmitted until they are acknowledged. The length of time between retransmissions is a configurable per-interface value, RxmtInterval. If this is set too low for an interface, needless retransmissions will ensue. If the value is set too high, the speed of the flooding, in the face of lost packets, may be affected.

                                        Action taken in state
        Circumstances          Backup                All other states
        _______________________________________________________________
        Advertisement  has     No  acknowledgment    No  acknowledgment
        been  flooded back     sent.                 sent.
        out receiving  in-
        terface  (see Sec-
        tion 13, step 5b).
        _______________________________________________________________
        Advertisement   is     Delayed acknowledg-   Delayed       ack-
        more  recent  than     ment sent if adver-   nowledgment sent.
        database copy, but     tisement   received
        was   not  flooded     from    Designated
        back out receiving     Router,  otherwise
        interface              do nothing
        _______________________________________________________________
        Advertisement is a     Delayed acknowledg-   No  acknowledgment
        duplicate, and was     ment sent if adver-   sent.
        treated as an  im-     tisement   received
        plied  acknowledg-     from    Designated
        ment (see  Section     Router,  otherwise
        13, step 7a).          do nothing
        _______________________________________________________________
        Advertisement is a     Direct acknowledg-    Direct acknowledg-
        duplicate, and was     ment sent.            ment sent.
        not treated as  an
        implied       ack-
        nowledgment.
        _______________________________________________________________
        Advertisement's LS     Direct acknowledg-    Direct acknowledg-
        age is equal to        ment sent.            ment sent.
        MaxAge, and there is
        no current instance
        of the advertisement
        in the link state
        database (see
        Section 13, step 4).

                 Table 19: Sending link state acknowledgements.

Several retransmitted advertisements may fit into a single Link State Update packet. When advertisements are to be retransmitted, only the number fitting in a single Link State Update packet should be transmitted. Another packet of retransmissions can be sent when some of the advertisements are acknowledged, or on the next firing of the retransmission timer.

Link State Update Packets carrying retransmissions are always sent as unicasts (directly to the physical address of the neighbor). They are never sent as multicasts. Each advertisement's LS age must be incremented by InfTransDelay (which must be > 0) when copied into the outgoing Link State Update packet (until the LS age field reaches its maximum value of MaxAge).

If the adjacent router goes down, retransmissions may occur until the adjacency is destroyed by OSPF's Hello Protocol. When the adjacency is destroyed, the Link state retransmission list is cleared.

Next: 13.7. Receiving link state acknowledgments

Connected: An Internet Encyclopedia
RFC 1583 - 13.6. Retransmitting link state advertisements